Solar cells should have the highest possible degree of efficiency for the conversion of solar radiation power into electrical current. This is determined by a plurality of factors, such as, inter alia, the purity of the starting material, the penetration of impurities during crystallisation from the contact surfaces of the crystals with the crucible into the crystal interior, the penetration of oxygen and carbon from the surrounding atmosphere into the crystal interior and also by the growth direction of the individual crystal grains.
A common feature of all known production methods in which a large quantity of molten silicon is solidified to form an ingot is the fact that heat is withdrawn from the crystal melt from its base and hence a crystal grows from the bottom upward. Due to the typically high rate of solidification and the absence of a seed crystal, the crystal does not grow as a monocrystal but is multicrystalline. A block is formed comprising a plurality of crystal grains of which each grain grows in the direction of the locally prevailing temperature gradients.
Now, if the isotherms of the temperature field in the silicon melt are not planar and do not run parallel to the base of the crucible, i.e. horizontally, no planar phase interface forms and the individual grains do not grow parallel to each other and vertically from the bottom upward.
This is accompanied by the formation of linear crystal imperfections even within the monocrystalline regions. These undesired crystal imperfections can be made visible as etched pits by etching polished surfaces (e.g. on silicon wafers). A high number of linear crystal imperfections as described above therefore results in a higher etch density.
The minimisation of the density of etch pits, which may be influenced by a plurality of factors, inter alia the establishment of a planar phase interface has been a well-known requirement for a long time. The density of etch pits is therefore a measure of the success in the achievement of a pillar-type growth of the Si grains by means of a planar phase interface. Since the establishment of the HEM method (heat exchange method) as the first method suitable for mass production, attempts have been made to avoid the drawback of an almost punctiform heat sink on the base of the crucible (as is known, for example, from U.S. Pat. No. 4,256,530) and to achieve a vertical heat flow from the top downward in the molten silicon.
There are therefore a variety of solutions, which aim, as a first step, to create a heat sink that extends over the entire surface of the crucible base (see, for example, EP 0 631 832, EP 0 996 516, DE 198 55 061). The present invention is based on the assumption that a planar heat sink of this kind is provided.
To produce solar cells as inexpensively as possible, there is a further requirement for the entire silicon ingot to be available for further processing if at all possible. However, the production process is subject to restrictions. This is due on the one hand to the inward diffusion of impurities from the crucible wall into the silicon melt while on the other hand the segregation results in an accumulation of impurities on the upper side of the silicon ingot so that it is regularly necessary to remove edges of the silicon ingot. A further restriction is represented by the generally rectangular basic shape of solar cells. This makes it necessary to cut the silicon ingot to the desired cross section. In this regard, it is desirable for the amount of wastage to be kept as low as possible.
The production of multicrystalline silicon from a melt consumes much energy. Therefore, there is a further requirement for the capacity of the smelting furnace to be used to the optimum degree and to have effective thermal insulation. For reasons of space, the base area of the melting crucible should occupy as much as possible of the base area of the smelting furnace.
Due to the high economic importance of the production of silicon as a starting material for the production of semi-conductors and semi-conductor components, a plurality of different approaches for growing silicon monocrystals or multicrystalline silicon are known from the prior art. For example, U.S. Pat. No. 4,256,530 discloses a method for growing a silicon monocrystal using a melting crucible with two-layer walls so that the silicon melt does not come into direct contact with graphite or elemental carbon that would otherwise diffuse rapidly into the silicon melt.
To obtain the lowest possible dislocation density in the crystal, during the crystal growth, care should be taken to ensure that the phase interface between solid and liquid is as planar as possible and runs transverse to the direction of crystallisation. This objective requires the radial heat radiation to be kept as low as possible. According to WO 01/64975 A2, to form a planar phase interface between the base of a melting vessel and its upper opening, a vertically extending axial temperature gradient is applied and measures are taken to avoid heat dissipation through the side walls of the melting vessel. To this end, all heating elements are enclosed in a jacket of insulating material surrounding the melting vessel as a way of preventing an undesirable and uncontrolled heat flow. To this end, a jacket of insulating material is disposed between the jacket heater and the crucible as an additional way of preventing a radial heat flow. This achieves a dominance of the axial temperature profile created by the upper heater and bottom heater.
EP 1 147 248 B1 discloses a device for producing a monocrystal by growing the monocrystal from a melt, wherein the furnace has a rotationally symmetrical design and wherein a wedge-shaped thermal insulation is provided around the melting vessel, viewed in the longitudinal direction of the vessel, with an insulating effect decreasing going from the upper heater to the bottom heater. As a result, heat losses close to the bottom heater are greater than those close to the cover heater. This supports a temperature gradient in the longitudinal direction of the melting vessel that is determined by different temperatures of the upper heater and bottom heater. The thermal insulation also significantly restricts the heat flow in the radial direction of the melting vessel resulting in the formation of planar phase interfaces.
DE 102 39 104 A1, corresponding to US 2004/0079276 A1, discloses a crystal growing furnace for a VGF method or vertical Bridgman method. Two jacket heaters or flat, planar heating devices are disposed around the melting vessel coaxially and vertically one above the other. In addition, measuring devices are provided to determine radial temperature differences in the space between the jacket heaters and the melting vessel. A regulator sets the heat output of the jacket heaters in such a way that the temperature difference measured in the radial direction becomes zero. In this way, planar phase interfaces are established resulting in the production of high-quality, low-dislocation silicon monocrystals.
According to the prior art, the heaters and the external contour of a crystallisation system for multicrystalline silicon are usually rotationally symmetrical, i.e. they have a circular profile. Since the usual square-shaped crucible is surrounded by this circular heater, the problem of corner overheating occurs. This results in thermal stresses causing flaking in the corners and consequently a comparatively large amount of wastage, which it is desirable to avoid. Typically, with the production of large fluoride monocrystals and of germanium crystals, round crystals are produced in round crucibles. The crucibles are surrounded by round heaters that display no differences between the upper and lower regions as far as thermal radiation is concerned.